Replaces uuid.uuid4 with uuidutils.generate_uuid()
Replaces uuid.uuid4 with uuidutils.generate_uuid()

As of now, UUID is being generated using either uuid.uuid4()
or uuidutils.generate_uuid(). In order to maintain consistency,
we propose to use uuidutils.generate_uuid() from oslo_utils.

Change-Id: I9db015450d8d932be297ec1cde854ee01f589b2e
Related-change: I620cb1f396ce011b9846ff2dad2c9811bc5d0652

openstack/mistral,openstack/mistral,StackStorm/mistral,StackStorm/mistral